Extension drilling typically involves the use of a drill bit mounted at the end of a drill string which is both rotated and subjected to longitudinal impacts. The upper end of the drill string is connected to an above-ground drilling machine which performs the rotation and imparts the impact. Such a percussive drilling technique is commonly referred to as bench drilling. The present invention can also be used in, for example, long-hole drilling, drifting and tunneling.
A general description of percussive drill strings comprising rods are addressed in, for example, U.S. Pat. No. 6,164,392 and U.S. Pat. No. 6,681,875. These are examples of the most used type of drill string in percussive drilling. However, these rods tend to reduce the drilling speed and drilling accuracy, as well as increase the risk of the bit becoming stuck in the ground. Prior rods are prone to overheating and subsequent failure of the thread joints.
A drill string comprising tubes is addressed in European Patent Number 126740. Tubes in comparison to rods are tubular in shape, i.e. the wall thickness is small as compared to the diameter of the tube. With the known solution the flushing properties improve and the elastic section modulus is increased, compared to conventional drill rods, leading to improved drilling accuracy. However, the increased elastic section modulus of the tubes leads to higher stresses in the joints. This has the effect that the joints are difficult to break. The larger outer diameter of the tube, compared to conventional drill rods, decreases the gap between the drill string and the wall of the hole being drilled, leading to problems with transportation of cuttings from the hole.
The present invention combines the advantages of rods and tubes into a new component.